1. Field of the Invention
This invention relates to a field emission element provided with field emission cathodes known as cold cathodes.
2. Description of the Related Art
When the electric field applied at a surface of a metal or semiconductor is as large as 10.sup.9 V/m, electrons pass through the potential barrier because of the tunnel effect, thus emitting out in a vacuum even at room temperatures. This phenomenon is called field emission. The cathode emitting electrons designed on that principle is referred to as a field emission type cathode (sometimes merely abbreviated as FEC).
Recently, flat, area field emission cathodes each formed of an array of micron-size field emission type cathodes have been able to be manufactured fully using the semiconductor machining technology. The element formed of a great number of FECs on a substrate is used as an electron source for a field emission display (merely abbreviated as FED), lithographic electron beam apparatuses, or the like by irradiating the fluorescent screen with electrons emitted from each emitter.
FIG. 5 is a plan view partially and schematically illustrating a conventional field emission element. FIG. 6 is a cross sectional view illustrating a conventional field emission element taken along the line VI--VI of FIG. 5. Referring to FIG. 5 and FIG. 6, numeral 31 represents a cathode conductor, 32 represents an island region, 33 represents a resistance layer, 1 represents a cathode substrate, 6 represents an insulating layer, 7 represents an emitter cone, and 8 represents a gate electrode.
This prior art is disclosed, for example, in Japanese Patent Laid-open Publication No. 7-153369 (Application No. 5-320923). The cathode conductor 31 and the island electrodes 32 are formed on the cathode substrate 1 such as a glass substrate through the step of patterning a conductive thin film of Nb, Mo, Al, or the like. The island electrodes 32 are surrounded by moats formed in the stripe cathode conductor. Thus the cathode substrate is shaped in a curbed pattern. The island electrode 32 and the cathode substrate 31 are separated from each other by a gap with a predetermined space and are completely isolated on the same plane. A line of cathode electrode is formed of the cathode conductor 31 and the plural island electrodes 32. In the field emission display, plural cathode electrode lines are formed in a stripe pattern and cross perpendicularly to the gate electrode lines 8.
As shown in FIG. 6, a resistance layer 33 made of, for example, amorphous silicon (a-Si) is formed on the cathode conductor 31 and the island electrodes 32. An insulating layer 6 made of a silicon dioxide (SiO.sub.2) is formed over the resistance layer 33. The gate electrode 8 made of Nb, Mo, Al, or WSi.sub.2 is formed on the insulating layer 6. A plurality of openings are formed through the gate electrode 8 and the insulating layer 6. An emitter cone 7 acting as an emitter electrode (emitter chip) made of Mo is formed on the resistance layer 33 within each of the openings. The tips of the emitter cones 7 are viewed from the anode electrode (not shown) through each of the openings.
In the above-mentioned structure, the emitter cones 7 are electrically connected to the island electrode 32 via the resistance layer 33. Each island electrode 32 is electrically connected to the cathode conductor 31 via the junction of the resistance layer 33. Since the gap between the gate electrode 8 and the tip of the emitter cone 7 is set to the order of submicrons, the emitter cone 7 can field emit electrons by applying a small voltage of several tens of volts. Moreover, since the pitch between the emitters 7 can be set to the order of 5 to 10 .mu.m, several tens of thousands to several hundreds of thousands emitter cones 7 can be formed on a single cathode substrate 1.
The anode substrate of transparent glass and the cathode substrate 1 are disposed opposite to each other at a predetermined distance. The space between the anode substrate and the cathode substrate 1 are evacuated to a vacuum degree. An anode electrode is formed on the anode substrate to collect electrons emitted from the emitter cones. The electrons impinge the fluorescent substance coated on the anode electrode to glow the fluorescent substance. Plural emitter cones 7 formed on one or plural island electrodes 32 correspond to one display segment. In a color display, the plural emitter cones 7 corresponds to one of red, green and blue colors forming one display segment.
In the conventional structure, the cathode electrode line is divided into the cathode conductor 31; and the island electrode 32 and the resistance layer 33 is formed. The reason is as follows:
Firstly, since the distance between the emitter cone 7 and the gate electrode 8 is very short, the emitter cone 7 may be short-circuited with the gate electrode 8 due to dust during fabrication. Plural cathode electrode lines in the X-direction and plural gate electrode lines in the Y-direction are formed on the cathode substrate 1 in a matrix pattern. Emitter cones 7 corresponding to several hundreds of dots are formed for one cathode electrode line. In order to emit electrons, a specific cone emitter 7 is selected by applying an input signal to the cathode electrode line and a positive voltage to a line of the gate electrodes 8 perpendicular to the cathode electrode line. In this structure, even if one emitter cone 7 is short-circuited, the corresponding cathode electrode line is completely disabled, thus resulting in line malfunction.
Secondly, gases released within the field emission panel at the time of an initial operation may cause a discharge between the emitter cone 7 and the gate electrode 8 or the anode electrode, so that the excessive current often destroys the cathode electrode. Moreover, some of a great number of emitter cones 7 tend to easily emit electrons, thus producing abnormal bright spot on the screen.
To avoid such phenomena, the resistance layer 33 is disposed between the emitter cone 7 and the cathode substrate 31. When a specific emitter cone 7 emits excessive electrons, the resistance layer 33 increases its voltage drop according to the increasing current to suppress the electron emission, so that a burst of electron emission can be prevented. Hence the resistance layer 33 can prevent current from locally concentrating to the specific emitter cone 7. This feature allows the yield in fabrication to enhance and the operation of the system to stabilize.
Thirdly, when emitter cones are directly formed on the resistance layer 33, without forming the island 32, the resistance between the cathode electrode 31 and each emitter cone 7 depends on the distance therebetween. That is, the emitter cone formed near the cathode conductor 31 has a low resistance value. The emitter cone distant from the cathode conductor 31 formed on the middle portion of the group of the emitter cones 7 has a high resistance value. This means that the emitter cone 7 adjacent to the cathode conductor 31 emits a large amount of electrons while the emitter cone 7 at the middle portion emits a small amount of electrons. Hence, the electron emission amount becomes uneven.
To overcome such problems, open spaces are formed in the cathode conductor 31. The island electrodes 32 isolated from the cathode conductor 31 are formed within the open spaces. Plural emitter cones 7 are formed on each of the island electrodes 32. This structure can uniform the resistance values between the cathode conductor 31 and respective emitter cones 7, thus equalizing the electron emission characteristics.